3d printable hard ferrous metallic alloys for powder bed fusion

ABSTRACT

The present invention relates to alloy compositions for 3D metal printing procedures which provide metallic parts with high hardness, tensile strengths, yield strengths, and elongation. The alloys include Fe, Cr and Mo and at least three or more elements selected from C, Ni, Cu, Nb, Si and N. As built parts indicate a tensile strength of at least 1000 MPa, yield strength of at least 640 MPa, elongation of at least 3.0% and hardness (HV) of at least 375.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/415,667, filed on Nov. 1, 2016, which is fullyincorporated herein by reference.

FIELD

The present disclosure relates to alloy compositions and 3D printingprocedures to provide for the formation of metallic parts withrelatively high hardness, tensile strengths, yield strengths, andelongation. The alloys also indicate the ability to form desirablephases, such as metal carbide and/or metal carbonitride phases, thatcontribute to such mechanical property characteristics.

BACKGROUND

Metal 3D printing processes provide a multitude of exceptional benefitssuch as the ability to produce highly complex parts with largely reducedpart production time. For these reasons 3D printing is of high value tomany industries. While many 3D printing processes for building metalparts exist, the most widely adopted processes are those that utilizesolid-liquid-solid phase transformations to build parts. These processesare commonly referred to as powder bed fusion (PBF), selective lasermelting (SLM), and electron beam melting (EBM), hereinafter theseprocesses are referred to as PBF.

While PBF is exceptionally versatile in its ability to produce complexparts from specific metal alloys, the process has been limited to beingable to produce parts from relatively few alloy steels such as 316L,17-4PH and maraging steel M300. Among these alloys, only M300 has ahardness that is considered sufficient to classify the alloy as a hardalloy (HV>370).

Expanding the material breadth of hard PBF steel alloys has met avariety of issues foremost among which is the occurrence of crackformation upon or after the printing process. Cracking of parts can becaused by a number of factors such as thermal stresses, hot cracking,and liquation cracking, and generally the potential for crackingincreases as the hardness of the built parts increases and the toughnessdecreases.

Numerous industries have a great deal of interest in utilizing PBF withhigher hardness materials (HV>370) for applications such as tooling,dies, molds, cutting tools, gears, filters, and bearings. In addition tohigh hardness these applications typically also require high strength,toughness, and corrosion resistance, low environmental health, lowsafety and stewardship risk, and low cost.

SUMMARY

A method of layer-by-layer construction of a metallic part comprisingsupplying an iron-based alloy in particle form including the elements Crand Mo wherein Cr is present at 10.0 wt. % to 19.0 wt. %, Mo is presentat 0.5 wt. % to 3.0 wt. % and at least three elements from C, Ni, Cu,Nb, Si and N, wherein C is present at 0 to 0.35 wt. %, Ni is present at0 to 4.0 wt. %, Cu is present at 0 to 5.0 wt. %, Nb is present at 0 to1.0 wt. %, Si is present at 0 to 1.0 wt. % and N is present at 0 to 0.25wt. %;

the balance of said alloy composition containing Fe; and

forming one or more layers of the alloy by melting the alloy into amolten state and cooling and forming a solidified layer of the elementswherein each of the solid layers has a thickness as formed of 2.0microns to 200.0 microns. The metallic part comprising one or morelayers has the following properties: tensile strength of at least 1000MPa, yield strength of at least 640 MPa, elongation of at least 3.0%,hardness (HV) of at least 375.

The present invention also relates to a 3D printed metallic partcomprising one or more iron based metallic alloy layers including theelements Cr and Mo wherein Cr is present at 10.0 wt. % to 19.0 wt. %, Mois present at 0.5 wt. % to 3.0 wt. % and at least three elements from C,Ni, Cu, Nb, Si and N, wherein C is present at 0 to 0.35 wt. %, Ni ispresent at 0 to 5.0 wt. %, Cu is present at 0 to 5.0 wt. %, Nb ispresent at 0 to 1.0 wt. %, Si is present at 0 to 1.0 wt. % and N ispresent at 0 to 0.25 wt. % and the balance of said alloy compositioncontains Fe;

said layers having thickness in the range of 2.0 microns to 200.0microns; and

said printed metallic part indicates a tensile strength of at least 1000MPa, yield strength of at least 640 MPa, elongation of at least 3.0%,and hardness (HV) of at least 375.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an optical image of alloy 1 (A10) built on a SLM 280 HLmachine.

FIG. 2 is an optical image of alloy 1 (A10) built on a Trumpf TRUMAFORMLF 250 PBF machine.

FIG. 3 is an optical image of alloy 1 (A10) built on an EOS machine witha density>99.5%.

FIG. 4 shows a 10,000×SEM micrograph of as-built alloy 1 (A10).

FIG. 5 shows the alloy 1 equilibrium phase diagram produced withThermo-Calc.

FIG. 6 shows microstructures at the surface of a part made of alloy 5(Table 1) at two different magnifications after carburization.

FIG. 7 shows hardness as a function of depth in a carburized casehardened alloy 5 and alloy 8 (Table 1).

FIG. 8 shows microstructures at the surface of a part of alloy 9(Table 1) at two different magnifications after nitriding.

FIG. 9 shows hardness as a function of depth in a nitrided case hardenedalloy 8 and 9 (Table 1).

DETAILED DESCRIPTION

A new class of steel alloys have been developed that combine excellentprintability with, in both the “as built” and in the “heat treated”state, high hardness (>375 HV), high yield and tensile strength, andhigh elongation as well as low safety (EH&S) and stewardship risk andrelatively low cost.

Printability of an alloy is defined as the ease of printing a metalalloy on a variety of commercial PBF machines without cracking orexcessive porosity in the built parts. The as-built condition isunderstood herein as the condition of the PBF built parts upon removalfrom the PBF machine, i.e. without any post-build heat treatment. Theheat treated condition is understood herein as the condition of the PBFbuilt parts that have been subjected to a post-build heat treatment. Thealloys herein are capable of 3D printing which refers to a process tocreate a three-dimensional object.

Table 1 below sets out the alloy chemistries that are preferablyemployed herein, which includes alloy 1 (A10) and then 10 additionalalloys for a total of 11 alloys:

TABLE 1 Alloy Chemistries (Wt. %) Alloy Fe C Cr Ni Cu Nb Mo Si N 1 (A10)84.93 0.16 10.64 1.96 0.54 0.03 1.48 0.19 0.07 2 84.92 0.25 11.5 1 0.50.03 1.5 0.25 0.05 3 84.97 0.2 11.5 1 0.5 0.03 1.5 0.25 0.05 4 84.960.16 11.5 1.05 0.59 0.04 1.51 0.18 0 5 84.57 0.21 11.21 1.93 0.12 0.031.7 0.19 0.04 6 81.66 0.21 15.55 0.88 0.55 0.03 0.89 0.16 0.07 7 82.820.21 11.69 0.94 2.56 0.06 1.47 0.17 0.08 8 86.85 0.1 10.56 0 0.56 0.041.61 0.14 0.14 9 85.37 0.17 11.01 1.85 0 0.04 0.95 0.53 0.074 10  86.640.12 10.55 0 0.55 0.08 1.52 0.39 0.14 11  87.03 0.11 11.03 0 0 0.08 1.380.2 0.16

Accordingly, it can be appreciated from the above that one supplies ametal alloy in particle form comprising, consisting essentially of, orconsisting of Fe, Cr and Mo, wherein Cr is present at 10.0 wt. % to 19.0wt. %, Mo is present at 0.5 wt. % to 3.0 wt. % and at least three ormore elements from C, Ni, Cu, Nb, Si and N, wherein C is present at 0 to0.35 wt. %, Ni is present at 0 to 5.0 wt. %, Cu is present at 0 to 5.0wt. %, Nb is present at 0 to 1.0 wt. %, Si is present at 0 to 1.0 wt. %and N is present at 0 to 0.25 wt. %. The balance of said alloycomposition contains Fe. Accordingly, one may select four elements, fiveelements or all six elements from C, Ni, Cu, Nb, Si and N for a givenalloy formulation.

In a preferred embodiment, one again supplies a metal alloy in particleform comprising, consisting essentially of, or consisting of Fe, Cr andMo, wherein Cr is present at 10.0 wt. % to 18.3 wt. %, Mo is present at0.5 wt. % to 2.5 wt. % and at least three or more elements from C, Ni,Cu, Nb, Si and N, wherein C is present at 0 to 0.30 wt. %, Ni is presentat 0 to 4.0 wt. %, Cu is present at 0 to 4.0 wt. %, Nb is present at 0to 0.7 wt. %, Si is present at 0 to 0.7 wt. % and N is present at 0 to0.25 wt. %. The balance of said alloy composition contains Fe.

Furthermore, the alloy may include some amount of inevitable impuritieswherein the level of such impurities may be up to 1.0 wt. %, Forexample, an element not listed above may also be present at a level ofup to 1.0 wt. %, where the corresponding level of Fe can then be reduced1.0 wt. %. With regards to impurities, it is noted that such iscontemplated to include elements such as sulfur, phosphorous and oxygen.

Alloy 1, which was previously designated as alloy A10, may itself havethe following preferred composition: Fe at 82.0 to 86.0 wt. %; Cr at10.5 to 12.0 wt. %; Ni at 1.5 to 2.5 wt. %; Cu at 0.4 to 0.7 wt. %; Moat 1.2 to 1.8 wt. %, C at 0.14 to 0.18 wt. %, Nb at 0.02 to 0.05 wt. %,N at 0.04 to 0.07 wt. % and Si at 0-1.0 wt. %.

The metal alloy is supplied to the PBF process in powder particle orwire form and is preferably produced using conventional melting witheither gas, centrifugal, atomization utilizing gases such as nitrogen orargon gas, or water atomization. Nitrogen gas melting and atomizationcan be used to increase the nitrogen content in the powder alloy. Thepowder particles can have a diameter in the range of 1 to 200 microns,more preferably from 3 to 70 microns, and most preferably from 15 to 53microns.

PBF parts are preferably built from the metal alloy herein usingcommercially available conventional PBF machines such as the SLM®280HLor EOS M-280 and a Trumpf TRUMAFORM LF 250. The parts are preferablybuilt in a nitrogen or argon atmosphere. Parts may be built on a metalsubstrate that is preheated up to 300° C., such as in the range of 100°C. to 300° C., and more preferably in the range of 20° C. to 200° C. Inaddition, no preheating of the substrate can be employed. For the PBFprocedure herein one may utilize one or a plurality of lasers orelectron beams with an energy density of 30 to 500 J/m³, more preferablyin the range of 50 J/mm³ to 300 J/m³ and most preferably in the range of60 J/mm³ to 200 J/mm³.

The metal substrate is preferably composed of the alloys 1-11 in Table 1or from other materials e.g. from type 304L stainless steel. The PBFprocedure herein contemplates a build-up of individual layers eachhaving a thickness typically in the range of 2.0 microns to 200.0microns, more preferably 5.0 microns to 150.0 microns, and mostpreferably 5.0 microns to 120.0 microns. Accordingly, a suitable rangeof thickness for the built-up layers is 2.0 microns and higher. Morecommonly, however, the thickness range for the built up layers(combination of individual layers) is from 2 microns to 800 mm and evenhigher depending upon the capability or requirements of a given printingprocedure.

Porosity and cracking in parts can negatively affect a number of partproperties including strength, toughness, and fatigue resistance. Assuch, it is desired for dense parts to minimize porosity and cracking inPBF parts. Porosity in parts is preferably less than 1.0%, morepreferably less than 0.5%, and most preferably less than 0.2% althoughsome large parts can tolerate higher porosity levels, such as a porosityof greater than 1.0% to 15.0%. Low porosity and no cracking in theas-built PBF parts with the metal alloys herein is evidenced in thecross-sectional optical micrograph images shown in FIGS. 1-3 which weretaken from parts built with alloy 1 (A10) on SLM 280HL and TrumpfTRUMAFORM LF 250 PBF machines, and an EOS M280/290, respectively. Theparts shown in FIGS. 1-3 were built on a substrate with no preheating toa height of 10 mm, using 0.040 mm thick layers, for a total of 250layers in the parts. Porosity is measured with optical image analysis at100× magnification and alloy 1 show porosity of less than 0.2%.

PBF parts are preferably heat treated after the parts are built to beable to achieve relatively high hardness, strength, and ductility.Achieving high hardness in-situ with building parts without cracking isrelatively difficult due to the thermal stresses and thermal fatigue inthe parts as they are built, combined with the typically low toughnessand ductility of high hardness alloys. PBF uses an energy source tocreate a small, quickly traversing, molten metal weld pool toselectively melt the powder in a powder layer, which then re-solidifiesadding the next layer in the part. The heat of the traversing weld poolis largely conducted into the part, which results in raising the overallpart temperature and providing relatively large temperature gradients inthe local vicinity of the weld pool. Large continuous and cyclic thermalstresses can arise in parts during PBF part building due to the thermalgradients and phase transformations in the parts. Parts thereforepreferably have sufficient strength, toughness, and ductility to resistcrack formation under the localized stress conditions and resist crackpropagation under the continuous and cyclic stresses.

“As-Built” alloy properties: Table 2 shows comparative mechanicalproperties of PBF parts produced with commercial PBF steel alloys andalloy 1 (A10) from Table 1 in the as-built condition (without apost-heat treatment). Properties of alloy 1 (A10) were measured on partsthat were PBF built on a substrate with no preheating to a height of 10mm, using 0.040 mm thick layers, for a total of 250 layers in the parts.Table 2 shows the increased hardness and strength of the metal alloyherein over the commercially applied crack-free steel alloys.

TABLE 2 Tensile Yield Strength Strength Elongation Hardness Alloy [MPa][MPa] [%] [HV] 316L 640 530 40 171 17-4PH 930 586 25 230 M300 1100 105010 332 Alloy 1 1504 1254 17 454 (A10)

With respect to the hardness data in Table 2, it is worth noting thatthe reported hardness is such that it is observed to be a function ofthe alloy composition as well as the printing procedure employed.Accordingly, in the case of, e.g. M300, the printing hardness may vary,depending upon the printing procedure, such that the HV hardness may bein the range of 320 to 370.

Table 3 below now provides the mechanical properties for all the alloysidentified in Table 1 in the “AB” or as-built condition, without a heattreatment and in condition “B1” which is reference to a heat treatment,which heat treatment is discussed further herein:

TABLE 3 Test Hard. YS UTS Elong. Alloy Condition (HV) (MPa) (MPa) (%) 1AB 454 1254 1504 17 (A10) B1 502 1325 1659 16 4 AB 450 1048 1082 3 B1491 995 1011 1 5 AB 531 1207 1659 19 B1 556 1338 1785 10 6 AB 381 6491422 19 B1 519 1329 1631 12 7 AB 415 1003 1538 20 B1 574 1571 1880 6 8AB 455 1041 1274 8 B1 561 1345 1629 13 9 AB 474 1274 1552 15 B1 531 13371676 14 10  AB 438 979 1191 7 B1 564 1368 1637 10 11  AB 501 1090 1365 7B1 563 1438 1621 5

As may therefore be appreciated from the above, the alloys herein, inthe as-built condition (no heat treatment) are such that they indicate atensile strength of at least 1000 MPa, more preferably at least 1100MPa, or at least 1200 MPa, and even more preferably, at least 1300 MPa.Moreover, it can now be appreciated that the tensile strength of the asbuilt alloys herein falls in the range of 1000 MPa to 1900 MPa, or 1100MPa to 1900 MPa, or 1200 MPa to 1900 MPa or 1300 MPa to 1900 MPa.

The above tensile strength is achieved in combination with a yieldstrength of at least 640 MPa, or at least 700 MPa, or at least 800 MPa,or at least 900 MPa, or at least 1000 MPa, or at least 1100 MPa, or atleast 1200 MPa, or at least 1300 MPa, or at least 1400 MPa or at least1500 MPa. Moreover, it can now be appreciated that the yield strength ofthe as built alloys herein falls in the range of 640 MPa to 1500 MPa.

Moreover, the above tensile strength and yield strength is alsopreferably achieved in combination with an elongation of at least 3%,4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, etc. up to 25%. Moreover, it can beappreciated that the elongation of the as built alloys herein fall inthe range of 3% to 25%.

The above tensile strength, yield strength and elongation is thenpreferably achieved in combination with a hardness (HV) value of atleast 375, 400, 410, 420, 430, 440, etc. up to 600. Moreover, it can beappreciated that the HV values of the alloys herein fall in the range of375 to 600.

Accordingly, it should be appreciated that the alloys herein are suchthat they can provide in the as built condition a tensile strength of atleast 1000 MPa, a yield strength of at least 640 MPa, and elongation ofat least 3% and a hardness (HV) value of at least 375. Othercombinations of tensile strength, yield strength, elongation andhardness may now be selected from the individual preferred levels oftensile strength, yield strength, elongation and hardness noted hereinfor the non-heat treated alloy.

FIG. 4 shows a 10,000×, secondary electron, scanning electron microscopy(SEM) micrograph of a PBF-produced, as-built, alloy 1 (A10) part. Thepart shown in FIG. 4 was built on a substrate with no preheating to aheight of 10 mm, using 0.040 mm thick layers, for a total of 250 layersin the parts. SEM imaging was performed on a Jeol JSM-7001F FieldEmission SEM. The microstructure in FIG. 4 is contemplated to containthe BCC/martensite, FCC, M₂CN, and M₇C₃.

FIG. 5 shows the alloy 1 (A10) equilibrium phase diagram produced withThermo-Calc showing the phase fraction of each phase that isthermodynamically stable over a temperature range from 20° C. to 1500°C. The equilibrium phase diagram was used to identify the phases withthe highest potential to contribute to increasing hardness and strength.

It is contemplated that the elevated temperature of PBF parts duringbuilding, which is caused by heat transfer to the part from thetraversing weld pool, may be sufficiently high in the metal alloysherein to drive in-situ precipitation of secondary phases such as theCu-rich FCC phase, the M₂N ((Cr,Mo)₂N) phase, and the M₂₃C₆((Cr,Fe,Mo)₂₃C₆) phase shown in the FIG. 5 phase diagram for alloy 1(A10). The in-situ precipitation of these phases during the part buildis expected to contribute to the part strength and hardness in theas-built condition.

“Heat Treatment”: PBF parts produced with the metal alloys herein can befurther enhanced by heat treating to increase the strength and hardnessof the parts. It is contemplated that various heat treatments can beperformed to affect the part properties and the heat treatmenttemperatures can be selected from equilibrium phase diagrams.

Effective heat treatments for the metal alloys herein are contemplatedto include (1) high temperature solutionizing (dissolving one or more ofthe secondary phases), quenching, and tempering (precipitation of thesecondary phases) and/or (2) tempering of the as-built part, with eachheat treating step being performed in a vacuum, argon, or nitrogenatmosphere. Solutionizing is preferably performed at a temperature ofgreater than 900° C., and for example in the range of 900° C. to 1400°C. and tempering is preferably performed at a temperature in the rangeof 150−900° C.

-   -   (1) The high temperature solutionizing and quenching step is        contemplated to:        -   a. reduce anisotropy in the part that can result from the            PBF process,        -   b. increase martensite content and thereby hardness and            possibly strength        -   c. dissolve Cr carbides and/or Cr nitrides that can            negatively affect the corrosion resistance of the part,        -   d. coarsen the undissolved carbides and/or nitrides.    -   (2) Further strengthening and hardening of the part via        additional precipitation of various phases is contemplated to be        initiated by subsequent tempering treatments.

“Heat Treatment”—procedure: The equilibrium phase diagram in FIG. 5 wasused to select solutionizing and tempering temperatures for PBF partsfrom alloy 1 (A10). The heat treatment used on the alloy 1 (A10) PBFparts consisted of solutionizing at 1000° C. for 1.5 hr followed by agas quench to −84° C. for 2 hours, and finally tempering at 454° C. for48 hr in Argon to strengthen and harden the part.

“Heat Treated”—alloy properties: Properties of the heat treated PBFalloy 1 (A10) parts are shown in Table 4 along with commercial PBF steelalloys after subjecting them to their manufacturer-prescribed heattreatments for PBF parts. Properties of heat treated alloys 1, 4, 5, 6,7, 9, 0, 10 and 11 were also listed in Table 3. Properties of alloy 1(A10) were measured on heat treated parts that were PBF built on asubstrate with no preheating to a height of 10 mm, using 0.040 mm thicklayers, for a total of 250 layers in the parts. The hardness of alloy 1(A10) shown in Table 4 was taken at the surface of the heat treatedpart.

TABLE 4 Tensile Yield Strength Strength Elongation Hardness Alloy [MPa][MPa] [%] [HV] 316L 843 587 28 262 17-4PH 1100 590 29 311 M300 2050 19902 544 Alloy 1 1659 1325 16 502 (A10)

As may therefore be appreciated from Tables 3 and 4, the alloys hereinfollowing heat treatment are such that they indicate a tensile strengthof at least 1000 MPa, or at least 1100 MPa, or at least 1200 MPa, or atleast 1300 MPa, or at least 1400 MPa, or at least 1500 MPa, or at least1600 MPa, or at least 1700 MPa, or at least 1800 MPa. Moreover, it canbe appreciated that the heat-treated alloys have a tensile strength inthe range of 1000 MPa to 1900 MPa.

Such tensile strength is achieved in combination with a yield strengthof at least 900 MPa, or at least or at least 1000 MPa, or at least 1100MPa, or at least 1200 MPa, or at least 1300 MPa, or at least 1400 MPa,or at least 1500 MPa, or at least 1600 MPa. Moreover, it can beappreciated that the heat-treated alloys herein have a yield strength inthe range of 900 MPa to 1600 MPa.

Such tensile strength and yield strength is also preferably achieved incombination with an elongation of at least 1%, 2%, 3%, 4%, 5%, 6%, 7%,8%, 9%, 10%, etc. up to 16%. Moreover, it can be appreciated that theheat-treated alloys herein have elongation values in the range of 1% to16%.

Such tensile strength, yield strength and elongation is then preferablyachieved in combination with a hardness (HV) value of at least at least475, or at least 500, or at least 525, or at least 550, or at least 600.Moreover, it can be appreciated that the heat-treated alloys herein haveHV values in the range of 475 to 650.

Accordingly, it should be appreciated that the alloys herein with heattreatment are such that they can provide, e.g., a tensile strength of atleast 1000 MPa, a yield strength of at least 900 MPa, and elongation ofat least 1% and a hardness (HV) value of at least 475. Othercombinations of tensile strength, yield strength, elongation andhardness may now be selected from the individual preferred levels oftensile strength, yield strength, elongation and hardness noted hereinfor the heat treated alloy.

Table 4 shows that heat treatment does not raise the hardness of 316Land 17-4PH to a level that either alloy could be classified as a hardalloy (HV>370). Only the hardness value of M-300 after heat treatmentclassifies the alloy as a hard alloy (HV>370) and M300 is currently theprimary alloy choice in additive manufacturing when a hard alloy isrequired. However, the application space of M-300 is highly limitedsince M300 features at such hardness levels indicate only a lowelongation (2%), indicating a tendency for parts to fracture or undergochipping when exposed to even small impact force such as dropping thepart to the floor. Therefore, the application of M300 finds relativelylimited industrial use. Additionally, the M300 alloy contains asignificant concentration of relatively high cost elements (18 wt % Ni,9 wt % Co, and 5 wt % Mo) and as such would not be considered a low-costalloy further limiting its industrial use. Finally, the industrial useof M-300 is further limited due to its potential EH&S and productstewardship risks given its high cobalt content. Cobalt is known to be ahealth risk upon inhalation and a stewardship risk due to itsclassification as a conflict mineral since it is mainly sourced from theRepublic of Congo.

In contrast, the heat-treated alloy 1 (A10) has numerous benefitscompared to the current incumbent M300. Alloy 1 (A10) has a higherhardness, a higher elongation, a lower cost structure, and is preferablycobalt free.

Case Hardening Treatment—

The surface hardness of PBF parts produced with the metal alloys hereincan be further enhanced by carburizing and nitriding case hardeningtreatments. These treatments introduce carbon and nitrogen,respectively, to the surface of the part, creating a case layer withincreased hardness relative to the “as-built” or “heat-treated”conditions while retaining the heat treated properties in the core. Itis contemplates that other treatments employed for case hardening suchas carbonitriding can also be used.

Carburizing—

The carburizing process for the metal alloys herein preferably includesa combination of the following steps: oxide reduction, carburizing,solutionizing, quenching, and tempering. Oxide reduction is performed ina reducing atmosphere at temperatures preferably between 800° C. and1200° C., more preferably between 900° C. and 1150° C., and mostpreferably between 950° C. and 1100° C. Carburizing is performed by amethod that provides or generates a source of carbon in the atmosphereor environment surrounding the part, such as pack, gas, vacuum, liquid,and plasma carburizing, at temperatures preferably between 800° C. and1000° C., more preferably between 850° C. and 975° C., and mostpreferably between 875° C. and 950° C.

The carburization results in an enrichment of carbon at the surface ofthe part resulting in a layer of material with a differentmicrostructure compared to that of the core as seen at two differentmagnifications in FIG. 6 for alloy 5. This structure results in amaximum hardness at the exterior surface that is preferably 650 to 1000HV, more preferably 700 to 975 HV, and most preferably 800 to 950 HV.The hardness then progressively decreases with increasing distance fromthe exterior surface (i.e. depth into the part) until it reaches asteady-state value in the core similar to heat treated values discussedherein. Representative examples of the hardness as a function of depthin carburized case hardened alloys 5 and 8 are seen in FIG. 7. Otheralloys listed herein can similarly be case hardening by a carburizingprocess with similar effectiveness. The level of carbon can be increasedat the surface down to a depth of at least 2.0 mm, and up to 4.0 mm.

Nitriding—

The nitriding process for the metal alloys herein includes a combinationof the following steps: solutionizing, quenching, and tempering. It iscontemplated that the nitrogen may be introduced to the surface of thepart by other nitriding methods, including plasma and liquid nitridingprocesses. The enrichment of nitrogen at the surface of the part resultsin a layer of material with a different microstructure compared to thatof the core as seen at two different magnifications for alloy 9(Table 1) are illustrated in FIG. 8. This structure results in a maximumhardness at the exterior surface that is preferably 700 to 1300 HV, morepreferably 750 to 1250 HV, and most preferably 825 to 1225 HV. Thehardness then progressively decreases with increasing distance from theexterior surface (i.e. depth into the part) until it reaches asteady-state value in the core similar to heat treated values discussedherein. Representative examples of the hardness as a function of depthin nitrided case hardened alloys 8 and 9 are seen in FIG. 9. As can beseen, the level of nitrogen is increased from the surface down to adepth of at least 200 am and up to 400 am. Other alloys listed hereincan similarly be case hardened by a nitriding process with similareffectiveness.

1. A method of layer-by-layer construction of a metallic partcomprising: supplying an iron-based alloy in particle form including theelements Cr and Mo wherein Cr is present at 10.0 wt. % to 19.0 wt. %, Mois present at 0.5 wt. % to 3.0 wt. % and at least three elements from C,Ni, Cu, Nb, Si and N, wherein C is present at 0 to 0.35 wt. %, Ni ispresent at 0 to 5.0 wt. %, Cu is present at 0 to 5.0 wt. %, Nb ispresent at 0 to 1.0 wt. %, Si is present at 0 to 1.0 wt. % and N ispresent at 0 to 0.25 wt. %; the balance of said alloy compositioncontaining Fe; and forming one or more layers of the alloy by meltingthe alloy into a molten state and cooling and forming a solidified layerof the elements wherein each of the solid layers has a thickness asformed of 2.0 microns to 200.0 microns; said metallic part having thefollowing properties: tensile strength of at least 1000 MPa, yieldstrength of at least 640 MPa, elongation of at least 3.0%, hardness (HV)of at least
 375. 2. The method of claim 1 wherein Cr is present at 10.0wt. % to 18.3 wt. %, Mo is present at 0.5 wt. % to 2.5 wt. %, C ispresent at 0 to 0.30 wt. %, Ni is present at 0 to 4.0 wt. %, Cu ispresent at 0 to 4.0 wt. %, Nb is present at 0 to 0.7 wt. %, Si ispresent at 0 to 0.7 wt. % and N is present at 0 to 0.25 wt. %, thebalance Fe.
 3. The method of claim 1 wherein said alloy comprises Fe at82.0 wt. % to 86.0 wt. %; Cr at 10.5 wt. % to 12.0 wt. %; Ni at 1.5 wt.% to 2.5 wt. %; Cu at 0.4 wt. % to 0.7 wt. %; Mo at 1.2 wt. % to 1.8 wt.%; C at 0.14 wt. % to 0.18 wt. %; Nb at 0.02 wt. % to 0.05 wt. %; N at0.04 to 0.07 wt. % and Si at 0 to 1.0 wt. %.
 4. The method of claim 1wherein said metallic part has the following properties: a tensilestrength of 1000 MPa to 1900 MPa, a yield strength of 640 MPa to 1500MPa, an elongation of 3.0% to 25.0%, and a hardness (HV) of 375 to 600.5. The method of claim 1 wherein said layers have a thickness of 2.0microns to 200 microns.
 6. The method of claim 1 wherein the melting isachieved by laser or electron beams with an energy density in the rangeof 30 J/mm³ to 500 J/mm³.
 7. The method of claim 1 wherein the metallicpart is built in a nitrogen and/or argon atmosphere.
 8. The method ofclaim 1 wherein said metallic part is built on a substrate that ispreheated to a temperature of less than or equal to 300° C.
 9. Themethod of claim 1 wherein the metallic part undergoes solutionizing at atemperature of greater than 900° C. followed by a gas quench andcooling.
 10. The method of claim 9 wherein said metallic part aftercooling is tempered at temperature at or above 150° C.
 11. The method ofclaim 9 wherein said alloy indicates a tensile strength of at least 1000MPa, a yield strength of at least 900 MPa, an elongation of at least1.0% and a hardness (HV) of at least
 475. 12. The method of claim 1wherein said metallic part is carburized to increase the level of carbonfrom the surface down to a depth of 4.0 mm.
 13. The method of claim 1wherein said metallic part is nitrided to increase the level of nitrogenfrom the surface down to a depth of 400 am.
 14. The method of claim 1wherein alloy includes at least four elements selected from C, Ni, Cu,Nb, Si and N.
 15. The method of claim 1 wherein said alloy includes atleast five elements selected from C, Ni, Cu, Nb, Si and N.
 16. Themethod of claim 1 wherein said alloy includes C, Ni, Cu, Nb, Si and N.17. A 3D printed metallic part comprising: one or more iron basedmetallic alloy layers including the elements Cr and Mo wherein Cr ispresent at 10.0 wt. % to 19.0 wt. %, Mo is present at 0.5 wt. % to 3.0wt. % and at least three elements from C, Ni, Cu, Nb, Si and N, whereinC is present at 0 to 0.35 wt. %, Ni is present at 0 to 5.0 wt. %, Cu ispresent at 0 to 5.0 wt. %, Nb is present at 0 to 1.0 wt. %, Si ispresent at 0 to 1.0 wt. % and N is present at 0 to 0.25 wt. % and thebalance of said alloy composition contains Fe; said layers havingthickness in the range of 2.0 microns to 200.0 microns; and said printedmetallic part indicates a tensile strength of at least 1000 MPa, yieldstrength of at least 640 MPa, elongation of at least 3.0%, and hardness(HV) of at least
 375. 18. The printed metallic part of claim 17 whereinCr is present at 10.0 wt. % to 18.3 wt. %, Mo is present at 0.5 wt. % to2.5 wt. %, C is present at 0 to 0.30 wt. %, Ni is present at 0 to 4.0wt. %, Cu is present at 0 to 3.5 wt. %, Nb is present at 0 to 0.7 wt. %,Si is present at 0 to 0.7 wt. % and N is present at 0 to 0.25 wt. %, thebalance Fe.
 19. The printed metallic part of claim 17 wherein said alloyincludes at least four elements selected from C, Ni, Cu, Nb, Si and N.20. The printed metallic part of claim 17 wherein said alloy includes atleast five elements selected from C, Ni, Cu, Nb, Si and N.
 21. Theprinted metallic part of claim 17 wherein said alloy includes C, Ni, Cu,Nb, Si and N.
 22. The printed metallic part of claim 17 wherein saidpart indicates a tensile strength of 1000 MPa to 1900 MPa, a yieldstrength of 640 MPa to 1500 MPa, an elongation of 3.0% to 25.0%, and ahardness (HV) of 375 to 600.